Nowadays, there is an increasing demand for a low profile of a surface light source device with the progress of a low-profile mobile device provided with the surface light source device. In order to reduce a thickness of the surface light source device, it is necessary to reduce a thickness of a light guide plate. However, even if the flat light guide plate can be reduced in thickness, there is a limitation to reducing a height of a light source including an LED. Therefore, in the case where the thin, flat light guide plate is used, the height of the light source is larger than a thickness of an end surface (a light incident surface) of the light guide plate, and the light source disposed opposite to the light incident surface of the light guide plate projects above from an upper surface of the light guide plate. When the light source projects above from the light guide plate, light emitted from the light source is not entirely incident to the light incident surface of the light guide plate, and the light partially leaks to the outside to degrade use efficiency of the light.
In order to solve the above problem, there has been proposed a use of a light guide plate, in which a light introduction part having a larger thickness than a thickness of a flat light guide plate body is provided at an end of the light guide plate body and an inclined surface inclined from a maximum thickness point of the light introduction part toward the end of the light guide plate body is provided in the light introduction part. For example, WO 2010/070821 discloses a surface light source device in which the light guide plate is used.
FIG. 1A is a schematic diagram illustrating a surface light source device 11A in which a light guide plate 13 having a step-difference structure is used. The light guide plate 13 includes a light guide plate body 14 having a substantially even thickness and a wedge-shaped light introduction part 15. Light exit patterns 17, such as a deflection pattern and a diffusion pattern, are distributed in a rear surface of the light guide plate body 14. An inclined surface 16 is provided in the light introduction part 15. The inclined surface 16 is inclined from the maximum thickness point of the light introduction part 15 toward the end of the light guide plate body 14. A thickness of an end surface (a light incident surface 18) of the light introduction part 15 is larger than a height of a point light source 12. In the surface light source device 11A in which the light guide plate 13 is used, the thickness of the end surface of the light introduction part 15 is larger than the height of the point light source 12, whereby the light emitted from the point light source 12 is efficiently taken in the light introduction part 15. The light taken in the light introduction part 15 is guided to the light guide plate body 14 and spread in a flat manner, and the light is reflected or scattered by the light exit pattern 17, and output to the outside from a light exit surface 19 of the light guide plate body 14.
However, in the surface light source device 11A in which the inclined surface 16 is provided in the light introduction part 15 as illustrated in FIG. 1A, an angle formed by the light guided in the light guide plate 13 and a lower surface of the light guide plate 13 increases because the light is reflected by the inclined surface 16. Therefore, a directional characteristic of the light guided in the light guide plate 13 is vertically widened. Accordingly, when the light exit pattern 17 has the same distribution as the case where the light introduction part 15 does not exist (that is, the light guide plate having the even thickness), the light is easily reflected or scattered by the light exit pattern 17, a region close to the point light source 12 becomes brighter in the light exit surface 19, and the light exit surface 19 brightens up unevenly.
In the case of a surface light source device 11B in which the thickness of the light guide plate body 14 is reduced as illustrated in FIG. 1B, a frequency of the incidence of the light guided in the light guide plate body 14 to the lower surface of the light guide plate body 14 increases. Accordingly, when the light exit pattern 17 has the same distribution as the thick light guide plate body 14, the light is easily reflected or scattered by the light exit pattern 17, the region close to the point light source 12 becomes brighter in the light exit surface 19, and the light exit surface 19 brightens up unevenly.
Like a surface light source device 11C illustrated in FIG. 2A, in order to fully use the light in the light guide plate 13, namely, in order to prevent the light in the light guide plate 13 from being lost due to the leakage from the end surface on the side located farther away from the point light source 12, the light exit surface 19 may be inclined such that the thickness of the light guide plate body 14 is reduced with increasing distance from the point light source 12. In this case, the angle formed by the light and the lower surface of the light guide plate body 14 increases every time the light guided in the light guide plate 13 is reflected by the light exit surface 19. Thus, when the light exit pattern 17 has the same distribution as the light guide plate 13 having the even thickness, the light is easily reflected or scattered by the light exit pattern 17, the region close to the point light source 12 becomes brighter in the light exit surface 19, and the light exit surface 19 brightens up unevenly.
In the case where the light exit surface 19 brightens up unevenly while becoming brighter near the point light source 12, a method for decreasing distribution density (number density) of the light exit pattern 17 on the point light source side is conceivable as a countermeasure as illustrated in a surface light source device 11D of FIG. 2B.
However, when the number density of the light exit pattern 17 is decreased on the point light source side, the light exit pattern 17 becomes sparse on the point light source side, the light exit surface 19 brightens up unevenly, and the light exit patterns 17 brighten up as dots to form a bright spot.
In order to make the light exit pattern 17 less visible, there is also a method for reducing a size of the light exit pattern 17 to increase the number density of the light exit pattern 17. However, the light exit pattern 17 currently has the size of about 20 μm, and production accuracy of the light exit pattern 17 is hardly obtained when the size of the light exit pattern 17 is further reduced.